Early determination of fetal sex is medically relevant when there is a family history of sex dependent or X-linked disorders. The presence of cell free fetal (cff) DNA in maternal plasma allows for fetal genotyping at an earlier time point than either amniocentesis or chorionic villus sampling with lower risk to the fetus (Lo et al., Lancet 350, 485-487 (1997)). However, the total amount and relative proportion of cff DNA in maternal plasma is lower in earlier stages of pregnancy. Because of low relative amounts of cff DNA during the first trimester, assays for fetal sex determination must have extraordinary sensitivity and specificity, and must be able to detect Y-chromosomal sequences at very low copy numbers.